Understanding neurogenesis is contingent upon elucidating cell cycle exit. An essential feature of all cell cycle transitions is their irreversibility. Once a commitment to enter a particular cell cycle phase is made, return to the previous phase is not possible. This is in part due to ubiquitin mediated proteolysis pathways that target substrates for proteasomal degradation. Ubiquitin mediated proteolysis pathways contain E1, E2, and E3 enzymes that regulate both the timing and fidelity of degradation events. One of the most important E3 enzymes is a multi-subunit complex named the Anaphase Promoting Complex, or APC. The APC controls both the metaphase to anaphase transition and mitotic exit. The APC is also active during cell cycle exit and differentiation of neuronal precursors. The APC degrades the ubiquitin ligase component Skp2, which allows p27kip1 levels to rise, thereby inducing cell cycle exit. The overall goal of this research is to test the hypothesis that ubiquitin mediated proteolysis controlled by the ubiquitin ligase the Anaphase Promoting Complex (APC) is required for cell cycle exit in the developing nervous system.